Printing inks utilize a vehicle as a carrier for the pigment. The vehicle is required to wet the pigment by displacing air at the pigment surface, it is also responsible for getting the pigment to the substrate and holding it there, as well as contributing greatly to the final gloss, setting and various other properties in the ink film. Traditionally, such vehicles have been made from solvents, vegetable oils, resins (both liquid and hard) and may include other components such as co-solvents, rheological modifiers, driers and anti-oxidants. The most commonly utilized vehicle for paste printing inks has been based upon petroleum distillates as the major solvent. It will be immediately apparent that inks produced utilizing petroleum distillates as a solvent suffer from at least two major drawbacks--petroleum is a non-renewable resource and such distillates emit organic vapors during the handling and drying processes. Currently, there is pressure on the printing industry both from regulatory agencies as well as from consumers to reduce both the reliance on a non-renewable resource such as petroleum distillate as well as the potential polluting solvent emissions from such printing vehicles.
There have been attempts to substitute vegetable oils such as linseed oil, soy oil, canola oil, or tung oil as solvents in printing inks in place of the petroleum distillate. Such vegetable oils are a renewable resource and demonstrate substantial reductions in organic vapor emissions. However, it has been found that the use of such vegetable oils restricts the possible formulations of the vehicles because the compatibility and/or the solubility of most hard resins in vegetable oils is much less than that of the more traditional petroleum distillates. Thus, the formulater is restricted in vegetable oil based systems in the possible choices for the components of the vehicle, in particular, the choice of possible hard resins. Vegetable oil based printing inks also tend to be either very slow setting and/or the gloss of the final printed surface is sub-standard when compared to conventional petroleum based systems.
In attempts to overcome the drawbacks of vegetable oil solvents, mixtures of the fatty acids which help make up such vegetables oils have been utilized. While the use of the fatty acids alone increases the solubility of the printing vehicle system tremendously, such printing ink vehicles also display an acid value of over 200 compared with conventional printing ink vehicles based upon petroleum distillate as the solvent which typically have an acid value of less than 25. These high acid value vehicles can cause livering of some types of pigments, a property which is highly undesirable.